Mechanisms of Pain and Immune Processes

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Abstract

Summary Overview: This translational research program addresses pathophysiological processes related to neuropathic pain and the potential confluence of chronic pain, autoimmunity, infectious diseases, and their intersection in human patients. Chronic neuropathic pain can affect any part of the body and can occur due to a variety of insults, infections, autoimmune, or metabolic disorders (e.g., diabetic peripheral neuropathy). We are testing the hypothesis that, in some patients, chronic pain is initiated and/or maintained by immunopathological processes related to autoantibodies generated against proteins in peripheral nerve, Schwann cells, or possibly components of the central nervous system. Autoantibodies are known culprits in certain large fiber paraneoplastic peripheral neuropathies and large fibers can also contribute to neuropathic dysesthesias. Where pain is a component, we hypothesize the presence of autoantibodies to proteins found in nerve endings arising from small diameter pain-sensing (nociceptive) C- or A-delta nerve fibers. To test the hypothesis that some patients with painful neuropathic conditions have an autoimmune component, we established a sensitive, quantitative, liquid phase luminescence assay that uses recombinant protein antigen-luciferase fusions as tracers upon expression in mammalian cells. The goals of this research are to (a) understand potential molecular and cell biological mechanisms underlying human chronic pain disorders, and (b) use this knowledge to devise new treatments and diagnostics for pain and other disorders to which it can be adapted. The methodology we established, the luciferase immunoprecipitation systems (LIPS) assay, robustly and sensitively detects antibodies in serum, plasma, cerebrospinal fluid, or saliva and has been used in over 1,300 published reports. Our early investigations evaluated a range of diseases and disorders that antibodies play a role in, including viral and bacterial infections with and without nervous system involvement and autoimmune disorders in subsets of patients that have nervous system symptomology. In many neural autoimmune disorders, the major autoantigens are frequently plasma membrane receptors or ion channels (Burbelo et al., 2016). Our aim is to determine if the LIPS assay can detect auto-antibodies to these proteins in appropriate chronic pain patients. For example, we completed a study on shingles (herpes zoster) and patients in which shingles evolved into a painful neurological disorder called post-herpetic neuralgia (PHN). We detected some neutralizing anti-cytokine autoantibodies in a subpopulation of Zoster patients, and, interestingly, all of the patients with anti-cytokine antibodies had PHN. This is important because it suggests that some patients with PHN may require additional intervention to control the disorder other than just analgesic medications. In a recent publication we demonstrated both HSV1 transcripts and HSV1 antibodies in post-mortem human trigeminal ganglion and whole blood, respectively. The correspondence between titer and transcript was 100%. The fact that we used RNA-seq allowed us to align the HSV1 reads to the HSV1 genome and all of the transcripts aligned with the HSV1 latency associated transcript (LAT), indicating that none of the subjects had viral re-activation at the time of death. We can use this approach as a component in a larger study of headache and migraine. One of the most compelling aspects of this project is its broad applicability and the progressive layering and evolution of the datasets. As we increase the number of test antigens, and assay across conditions and diseases, we assemble comprehensive evaluations of immune and autoimmune responses. This is accomplished by determination of (a) the extent and specificity of immune response to orthologous proteins and protein fragments, (b) overlap in antigen profiles indicative of common denominators or general mechanisms, and (c) antigenicity within an entire signaling pathway involved in inter- or intracellular communication. Eventually, full multiple antigen profiling can be implemented to obtain a deeper level of understanding of the immune component of many complex human disease states. Our recent review summarizes our current thinking on pathogenic autoantibodies. Nearly all autoimmune disorders for which the autoantibody has a pathogenic action show that the antibodies target either a secreted peptide or protein, or the extracellular domain of an integral plasma membrane protein. The latter are usually transmembrane signaling proteins. We can now focus on the extracellular domains of transmembrane ion channels and receptors and enzymes in the search for antigenic targets. This concept greatly simplifies the search for antigens that may participate in generating or sustaining a chronic neuropathic pain condition.